You can see if this shared 4.91cm segment(if kits are on Gedmatch) between your son and match is located on his maternal chromosome by running Gedmatchs one to one using his maternal phased Data kit number and matches kit number. This may be on his maternal chromosome, but it also maybe a false segment created by combining values from his paternal and maternal chromosome. Your son has an extra 4.91cm/697SNPs segment at the beginning of this shared between the three of you. It is also why people say segments under 5cM-7cm/500-700 SNP can be false segments. This is where Gedmatch comes in handy, if all kits are uploaded there (yours and matches) His is shown as a 4th cousin and mine as a 5th cousin. If you've inherited an entire chromosome from one grandparent, there's a good chance that you'll end up sharing a large portion of it with one or more of your matches.Thank you Michael.Ĭhromosome Start LocationĞnd Location centiMorgans (cM) # of Matching SNPs This has been seen when people have tested their parents and one or more of their children. So, you may have your entire chromosome 8 from your paternal grandmother, with none from your paternal grandfather.
It also can happen that you inherit an entire chromosome from one of your parents' parents. If you've inherited an entire chromosome from one grandparent, there's a good chance that you'll end up sharing a large portion of it with one or more of your matches. But we can't tell how large a segment from a specific ancestor couple is unless one of our cousins has kept most of the same segment intact too, so that we know from which ancestor couple it came. The segments from a specific grandparent or great-grandparent are undoubtedly larger than what's noted here, except perhaps for the huge one I share with my 2nd cousin. However, this is on my x chromosome, which undergoes less recombination than chromosomes 1-22. I share a 47.5 cM segment with a paper trail proven 6th cousin. My sister shares one segment of 24.6 cM with an estimated 3rd to distant cousin. My sister shares one segment of 26.2 cM with another estimated 3rd to 6th cousin. We also share a single segment of 26.5 cM with another estimated 3rd to 6th cousin. My sister and I share one segment of 37.4 cM with an estimated 3rd to 6th cousin. This means that both she and I received this huge segment through 3 generations of recombination - our common great-grandparents to my grandfather and her grandmother and then from my grandfather to my father and her grandmother to her father and, finally, from my father to me and from her father to her. That represents about 65% of that chromosome, according to the figures on the page I linked to. My known 2nd cousin and I share a 121 cM segment on chromosome 7. Here are some examples from my sister's and my matches at 23andMe: For that to happen, both lines of descent must preserve the segment from the common ancestor. The only way you'd know about how large a specific segment is is if you and a relatively close cousin (1st or 2nd) match along most of the segment. Since chromosome 3 contains 219.1 cM (see ), this means that if there are 3 different segments on it from your grandparents, the average segment will be about 70 cM. See for an explanation of crossover points. This means that, for instance, the chromosome 3 that you received from your father may contain just a few large segments alternating from his mother and father. I think that the estimate for recombination crossover points on chromosomes is that there are usually only 3 or 4.
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